The Multimedia Broadcast/Multicast Service (MBMS) was standardized in release 6 version of the WCDMA system and continuously evolves during the Long Term Evolution (LTE) and Long Term Evolution-Advanced (LTE-A) standardization process. The MBMS is a point to multiple points information transmission service for transmitting a same multimedia information to a group of users simultaneously and includes a multimedia broadcast service and a multimedia multicast service. During the standardization process of LTE, it is required that a frequency utilizing rate of 1 bit/s/Hz can be achieved at cell borders in a broadcast mode.
One main reason limiting the spectrum efficiency of the MBMS is a “bottleneck problem” in the signal transmission during broadcast or multicast. FIG. 1 is a schematic diagram showing a multicast according to prior arts. Users receiving the MBMS are distributed in different locations of a cell and the Signal to Interference plus Noise Ratio (SINR) of the users at the center of the cell is high and the SINR of the users at the border of the cell is low. In order to guarantee that users at the cell border can correctly receive the signal, the signal of the MBMS can only be modulated with a comparatively low order modulation mode, such as QPSK (Quadrature Phase Shift Key). This feature makes the users with comparatively good channel quality at the center of the cell being not able to use a high order modulation mode, and thus limits the frequency utilizing rate of the whole system.
The hierarchical modulation technology is a comparatively good method to solve the bottleneck problem in the MBMS transmission. In the current communication systems, the hierarchical modulation can be used to transmit two or more independent data blocks on a same carrier resource and the user may select to receive a corresponding data block according to the channel quality. Since channel qualities are different, when data are wrongly received, data blocks to be retransmitted to different users are different. The conventional retransmission methods do not take into account the feature of retransmitting the broadcast or multicast data with the hierarchical modulation technology and the transmission efficiency is low.
The hierarchical modulation adopts a non-even modulation constellation map. FIG. 2 shows a constellation map according to the prior art. Under the QPSK+16QAM mode, the data to be transmitted is divided into two groups: a basic layer (BL) and an enhanced layer (EL) and data at different layers have different transmission qualities, wherein the data at BL equal to be QPSK modulated and the data at EL equal to be 16QAM modulated. Thus, users with comparatively bad channel qualities at the cell border may only demodulate the data at the BL and users with comparatively good channel qualities at the cell center may demodulate the data at the BL and the EL simultaneously. FIG. 3 is a schematic diagram showing a hierarchical modulation according to the prior art. The bottleneck problem in transmission may be appropriately solved by applying the hierarchical modulation technology to the MBMS. In FIG. 3, BS represents a base station and the hierarchies are 64QAM, 16QAM and QPSK. In addition, it is found that the hierarchical modulation technology has been broadly adopted in the existing broadcast systems, such as DVB-T (Digital Video Broadcasting—Terrestrial), DVB-H (Digital Video Broadcasting—Handheld), UMB BCMCS (Ultra Mobile Broadband Broadcast/Multicast Services), and the like.
The retransmission technology is a critical technology guaranteeing the reliability of the information transmission and plays an important role in the MBMS transmission. The conventional retransmission methods of MBMS include retransmission with a fixed constellation map, retransmission by exchanging the BL and the EL, retransmission by reconfiguring a constellation map and retransmission by combining the constellation reconfiguration and exchange of the BL and the EL.
1. Retransmission with a Fixed Constellation Map
FIG. 4 is a schematic diagram showing the method for retransmitting with a fixed constellation map according to the prior art. The constellation map used at the retransmission (RETX) is the same as that used at the first transmission (1stTX). In FIG. 4, UE represents mobile terminal, vertical line part represents the BL, dot part represents the EL, blank part represents there is no data, “√” represents the data is received correctly, “x” represents the data is wrongly received and the same representation will be used hereinafter. According to this method, since users at the center of the cell have good channel qualities and are more likely to receive the data at the BL correctly, the retransmission of the data at the BL has little meaning for users with good channel qualities. Therefore, according to this method, users at the cell center have low spectrum utilizing rate.
2. Retransmission by Exchanging the BL and the EL
FIG. 5 is a schematic diagram showing the method for retransmitting by exchanging the BL and the EL. When retransmitting, data at the BL and the EL are exchanged. For the users at the cell center, since the EL data requires higher channel quality over the BL data, this method brings obvious advantages. However, since the users at the cell border may only expect to receive the BL data, this method will cause a decrease in performance for the users at the cell border.
3. Retransmission by Reconfiguring the Constellation
Taking the method for retransmitting by reconfiguring the 16QAM modulation constellation as an example, different (four) constellation maps are used during retransmission in order to achieve the object of constellations diversity. As compared with method 2, this method can only improve the performance of the users at the cell border and does not contribute much to that of the users at the cell center.
4. Retransmission Combining the Constellation Reconfiguration and Exchange of the BL and the EL
In order to improve the performance of users at the cell border and at the cell center simultaneously, this method adopts a retransmission mode combining method 3 and 2 and alternatively adopts method 2 and 3 at different retransmission time. The problem of this method is that multiple retransmissions are needed for improving the channel qualities of the users at the cell border and at the cell center and thus the retransmission efficiency is comparatively low.
In summary, when a hierarchical modulation or a non-even constellation map is adopted, the users at the cell center and the cell border receive packets with different quality guarantees and the correctness in receiving packets by the users at the cell center and the cell border are different. In this case, how to arrange the retransmission signals in order to coordinate the retransmission of the BL data and the EL data and at the same time to improve the performance of the users at the cell center and the cell border has become the urgent need to be solved.
The above methods are incorporated by reference.